The present invention relates to an improved process for the recovery of polymer product from polymerisation processes and in particular to the recovery of polymer product from gas phase fluidised bed processes for the polymerisation of olefins. The present invention is most suitable for the recovery of polymer product from gas phase fluidised bed processes for the polymerisation and copolymerisation of ethylene and alpha-olefins.
Homopolymers of ethylene and copolymers of ethylene and α-olefins have been prepared commercially for many years in both batch and continuous processes in solution, slurry and gas phase reactors.
One particularly important gas phase process employs a fluidised bed reactor for the catalytic polymerisation of gaseous monomers and comonomers to produce solid particulate polymers and copolymers. Fluidised bed reactors may be operated by the continuous feed of catalyst and a gaseous stream containing the polymerisable monomers and comonomers to a fluidised bed composed of particulate polymeric material. The polymeric bed is kept fluidised by a rising stream of the gaseous feed mixture containing the monomers and comonomers. Unreacted monomers and comonomers and carrier gases which may be employed are carried out of the reactor and recycled and returned to the bottom of the reactor where additional feed gases are introduced. This combined mixture of gases is passed through the fluidised bed to maintain it in a fluidised state and to maintain the polymerisation reaction. The exothermic heat of the polymerisation reaction is removed by passage of the recycled gaseous stream through heat exchange equipment prior to its return to the bottom of the reactor. The polymer product from the reactor may be conveniently removed by conventional means.
The polymer product removed from the reactor may contain unreacted monomers, comonomers and other associated hydrocarbon species (hydrogen, ethane, methane, propane, pentane, hexane, butane) and these monomers, comonomers and the other hydrocarbons should be removed from the polymer product since failure to do so may lead to (a) hydrocarbons levels rising to explosive levels in downstream equipment or (b) environmental constraints being exceeded.
There are a number of prior art patents which describe methods for the removal of such monomers. For example U.S. Pat. No. 4,372,758 describes a degassing process which uses an inert gas such as nitrogen for the removal of unreacted gaseous monomer from the polymer product. Solid polymer is conveyed to the top of a purge vessel by means of an inert gas system, an inert gas stream is introduced into the bottom of the purge vessel and the solid polymer is counter currently contacted with the inert gas stream to remove the unreacted gaseous monomers from the solid polymer product.
The unreacted monomers may then be mixed with an inert gas stream which is often passed to a flare for disposal or vented to the atmosphere.
EP 127253 describes a process for the removal of residual monomers from ethylene copolymers by subjecting the copolymer to a reduced pressure zone sufficient to desorb the monomer, sweeping the copolymer with reactor gas which is free of inert gases and recycling the resultant gas containing the desorbed monomer to the polymerisation zone.
EP 683176 describes a process whereby the gas mixture comprising polymer product and unreacted monomers is passed from the reactor to a depressurization zone, separating the mixture into a solid phase and a gaseous phase, the solid phase then being subjected to a non-deactivating flushing with an inert gas and then a deactivating flushing with a gaseous mixture of nitrogen, water and oxygen. The gaseous phase is sent directly or indirectly back to the reactor.
EP 596434 describes a process for removing unpolymerised gaseous monomers from a solid olefin polymer containing gaseous monomers by counter currently passing a purge gas through said polymer in a purge vessel having upper and lower portions and a gas permeable, solids impermeable membrane between said portions.
U.S. Pat. No. 5,376,742 describes a process for minimizing the venting or flaring of the unreacted monomers whereby effluent gases are used to purge unreacted monomer from the polymer product. The process described therein also provides a process for the recycling of unreacted monomers to the reactor vessel as well as providing a process which minimizes or reduces the quantity of inert gases used for both the transport of polymer product and the purging or degassing of the unreacted monomer from the polymer product. In the process described in U.S. Pat. No. 5,376,742 a small portion (1-5%) of the recycle stream exiting the reactor is cooled and separated into a gas and a liquid stream. The gaseous stream comprising the majority of the ethylene as well as hydrogen and nitrogen is used for purging unreacted monomers from the polymer product in a degassing vessel. This gaseous stream is passed counter currently with the polymer product and unreacted monomers. A portion of the ethylene may be removed from the gaseous stream, prior to its use for the purging of the polymer product, and passed to an ethylene recovery unit. The gaseous stream together with the unreacted monomer purged from the polymer product is passed either to a flare for disposal or more usually through a recycle compressor and back to the polymerisation reactor.
A concern with the use of a gaseous stream comprising a major proportion of ethylene as the purge gas for degassing the polymer product is the potential for further reaction of the still active polymer powder present in the degassing vessel which may lead to blockages and a change in powder physical properties if allowed to occur. This is also a potential problem during the copolymerisation of ethylene and alpha-olefins, in particular hex-1-ene, the presence of which may lead to agglomerates in the degassing vessel.